Coolant brines based on aqueous solutions of organic salts have a markedly lower viscosity than brines based on ethylene glycol or propylene glycol, in particular at low temperatures in the range from -20 to -40.degree. C. A further advantage is their physiological acceptability, such that they can be preferably employed in the foodstuffs sector for indirect cooling (for example freeze-drying, deep-freeze cabinets). A high water content of over 40% by weight renders the brines nonflammable. Due to the low viscosity, considerable energy savings can be achieved by recirculation. Due to the high water content, advantageous heat technology and refrigeration properties are additionally achier ed, for example a high specific heat and a high heat transfer coefficient.
DE-A44 12 954 (EP-A-0 677 563) proposes aqueous potassium formate solutions, which contain up to 55% by weight of formate and are utilizable to -55.degree. C., as coolants. As an inhibitor, these brines contain 1,2,4-triazole and/or borax. In the absence of borax, a pH indicator, preferably phenolphthalein, is added for control of the pH. DE-A41 07 442 describes functional fluids, which essentially contain potassium acetate and potassium carbonate as an aqueous solution and which are inhibited against corrosion using small amounts of potassium fluoride, alkali metal salts of silicic acids and known inhibitors, for example benzotriazole, as heat transfer media and coolants.
A great problem of the cooling fluids mentioned is the corrosion of metallic materials caused by oxygen, in particular the corrosion of ferrous metals (iron, nickel, cobalt). Even if the cooling system is carefully flushed with nitrogen, in practice it can hardly be avoided that in the relatively long term air forces its way in again. The customary inhibitors from the silicates, phosphates and aromatic triazoles group in the long term are not effective enough in order to protect components made of ferrous metals, in particular of steel or gray iron.
Whereas concentrated aqueous solutions having a content of more than 40% by weight of potassium formate or potassium acetate, which contain the inhibitors mentioned, as a rule have a less corrosive action, this does not apply to dilute solutions. On increasing dilution with water, formic acid is formed from formates and acetic acid is formed from acetates by hydrolysis, which finally leads to a fall in the pH value and an increase in the corrosion rate.